The invention relates to a process for producing a deep-drawn article from a partially-crystalline polymeric material, i.e., one having crystallites and, more particularly, a process in which the material is provided in film form, heated to a temperature below the melting point of its crystallites, embossed with a surface pattern while so heated, cooled, crosslinked, reheated, this time to a temperature above the melting point of its crystallites, and deep drawn into the shape of the article.
Deep-drawn articles are often used as an interior finish or lining in motor vehicles. These are generally made from sheet material deep drawn into the shape of the finish or lining article.
The sheet material heretofore generally consisted substantially of polyvinyl chloride (PVC) or a mixture of acrylonitrile-butadiene-styrene copolymers (ABS) with PVC. A variety of modifications of such sheet material are made with soft or hard PVC. Such sheet material, or laminates thereof, is generally used as a film, meaning herein a sheet thickness of from about 500 micrometers to about 1400 micrometers and, preferably, of from about 600 micrometers to about 1200 micrometers. Such sheet material has a number of disadvantages.
If the sheet material was plasticizer free, like films of ABS and hard PVC, for example, it had the disadvantages in deep drawing and use of excessive stiffness and too low a flexibility at low temperatures, for example. These were sufficient that plasticizer was generally used.
If the sheet material contained plasticizers, however, the plasticizer tended to separate from the sheet material over time, especially if heated. In use, when heating could not always be controlled, the separated plasticizer condensed on the windshield of the motor vehicle, resulting in so-called "fogging", which clearly was undesirable. The separation of the plasticizer also gradually reduced the elongation characteristics of the sheet material in so-called heat aging which could impair its flexibility for deep drawing and, especially at low temperature, use.
The above-described disadvantages of plasticized sheet materials increase as the instantaneous and/or time-average ambient temperature about the sheet material increases. Such results for fogging and heat aging, as well as further disadvantages from similarly-reduced ozone resistance and light stability, can be measured.
Nevertheless, in the motor vehicle industry at the present time, especially for passenger cars, there is a trend toward lowering the air drag coefficient as much as possible for fuel economy. This has led to, among other things, a very low angle of inclination of the front windshield. Solar irradiation then heats the passenger compartment more to affect more deep-drawn interior linings and finishes therein as described and, especially, a dashboard lining which is particularly desirable to deep draw. The plasticizer problem has become, consequently, more pronounced than heretofore.
Accordingly, prior efforts have been made to avoid the known disadvantages of plasticized PVC film and like sheet material for deep-drawn articles, such as for example, by improving aging resistance, providing a higher degree of softness without the occurrence of fogging, limiting the loss of elongation characteristics over the useful life time of the material, providing good light stability, and increasing ozone resistance. Any of these would improve the deep-drawing characteristics of the material for motor vehicle use.
Some of the prior efforts propose the use of conventional PVC-ABS mixtures and attempt to eliminate the above-stated problems by adding polymeric plasticizers and other modifying resins, such as, for example, styrene-acrylonitrile copolymer (SAN) and/or acrylonitrile-butadiene resin (NBR). Such a proposal is to be found, for example, in German Federal Utility Model publication No. G 82 20 682.
It is also known to use partially-crystalline polymeric materials, i.e., those having crystallites, such as polypropylene, ethylene-propylene-diene copolymers (EPDM), ethylene-propylene copolymers (EPM) and polyamides, for example, in the automotive industry. Such materials can be used uncrosslinked by injection- or blow-molding processes, for example, or crosslinked by pressing processes, for example. In all the processes of making automotive products therefrom, at least generally, however, temperatures above the melting point of the crystallites of the material are reached.
Films of crosslinked or uncrosslinked EPDM and EPM copolymers would satisfy some of the above-described requirements for making deep-drawn automotive articles. They have good light stability, good cold flexibility, low fogging and good impact strength, for example. They have been used, at least primarily, however, only in the packaging industry in which, however, deep-drawn products are also made.
In deep-drawn packaging products from such films, the temperature of the material is always below the crystallite melting point in order to avoid decomposing the film. Moreover, the packaging and other products deep drawn from such films have not been embossed, as desired for a lining for a motor vehicle interior, and for economic and technical reasons understood in the known art, it is highly problematic that such could be done.
Economically, this is because a deep-drawing die having a surface texture corresponding to the texture to be embossed on the surface of the product would be required. This involves extremely high die-making and die-repairing costs (as well as technical difficulties for die repairs particularly). Moreover, all of the positive deep-drawing process equipment now used almost exclusively in manufacturing products for the interior of motor vehicles would have to be converted to equipment for the more-complicated negative process. In addition to the expense of new, more-complicated negative process equipment, products requiring great stretching and/or deep undercutting are more difficult to make on negative process equipment and this is often inimical to the styling concerns of automotive product designers.
Technically, the high, sun-produced temperatures which can occur at the dashboard, for example, of a modern, low-drag automobile having a windshield with low rake angle can produce considerable shrinkage if it is made from a sheet of partially crystalline material deep drawn below its crystallite melting point, as heretofore practiced. This is especially true if the sun-produced temperature of the dashboard reaches or nears the temperature of the material during the deep-drawing process. Such shrinkage can result in an undesirable deformation of the dashboard, even if it is foam backed.
In the case of moldings consisting of a mixture of PVC and ABS, attempts have been made to solve the solar-deformation problem by seeking, in the composition of the mixture, a compromise between good flow characteristics in the deep-drawing process, on the one hand, and high temperature stability and hardness of the sheet material used, on the other hand.
It is also known, from German patent publication AS No. 31 07 907, for example, that webs of partially-crystalline plastics including, especially, for example, EPDM and EPM copolymers, can be shaped into a product and then wholly or partially be crosslinked for fixing by "freezing." This process is applied in the production of so-called "shrinkables" utilizing the "elastic memory" of the crosslinked, partially-crystalline material, however, because a product so made rapidly shrinks back to the original shape and dimensions of the material when even briefly heated above the melting point of the crystallites of its material. This is not desirable, as before described, for a deep-drawn molding used as an interior finish or lining of an automobile.